Susan has a beaker with one mole of water and another beaker filled with one mole of hydrogen chloride. Compare the number of molecules that Susan has of each
substance.

Ratio is 1:1 ; they have same number of molecules.

Explanation:

One mole of any compound have same number of molecules, irrespective of their chemistry and composition, i.e. 6.022x10^23 . This number is known as Avogadro's Number.

So the beaker with one mole of water contains = 6.022x10^23 molecules ;

and beaker with one mole of Hydrogen chloride = 6.022x10^23 molecules.

The comparative ratio is 1:1.

if you mean hydrogen and oxygen then water is the product

the ideal gas law equation is as follows

pv = nrt

where p - pressure

v - volume

n - number of moles

r - universal gas constant

t - temperature

so if the pressure, volume and temperature are already known

we are left with n and r

since r is the universal gas constant that has a known fixed value then r too is known

so we are left with 'n'

once we know temperature volume and pressure

we can find the number of moles of gas present using the ideal gas law equation

this is not my own answer, this is an answer for the same question by the user blahblahmali. i do not claim ownership of this account.